Prognostic Meteorological Discussion
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FXAK02 KWNH 271856
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
255 PM EDT SAT MAY 27 2017

VALID 12Z WED MAY 31 2017 - 12Z SUN JUN 04 2017

THERE IS REASONABLE AGREEMENT ON THE LARGE SCALE PATTERN AT LEAST
FROM THE PERSPECTIVE OF RECENT ENSEMBLE MEANS, WITH AN AXIS OF
RELATIVELY LOWER HGTS ALOFT (CONTAINING ONE OR MORE EMBEDDED UPR
LOWS) FROM THE ALEUTIANS/BERING SEA INTO NERN PACIFIC WHILE
CANADIAN RIDGING EXTENDS OVER A SIGNIFICANT PORTION OF MAINLAND
ALASKA.  HOWEVER THE FULL SPECTRUM OF INDIVIDUAL ENSEMBLE MEMBERS
AND OPERATIONAL MODEL RUNS UP TO THIS POINT WOULD SUGGEST
CONSIDERABLE UNCERTAINTY WITH SPECIFICS ACROSS MUCH OF THE NRN
PAC/ALEUTIANS INTO BERING SEA WITH GUIDANCE SPREAD EXTENDING INTO
THE MAINLAND AND PANHANDLE AS WELL.

AS OF DAY 4 WED THERE IS DECENT MODEL AGREEMENT ON A CNTRL
ALEUTIANS SYSTEM FOR WHICH THE 06Z GEFS IS THE ONLY ENSEMBLE MEAN
THAT HAS SIMILAR DEFINITION.  MEANWHILE THERE IS ALREADY A
MODERATE AMOUNT OF SPREAD FOR THE SYSTEM TO THE S/SE OF KODIAK
ISLAND.  FOR THIS EARLY PART OF THE FCST AN OPERATIONAL MODEL
BLEND, MORE 12Z GFS/00Z ECMWF RELATIVE TO UKMET/CMC RUNS,
REFLECTED CONSENSUS IDEAS WHILE RESOLVING POSN DIFFS FOR THESE
SYSTEMS.  NOTE THAT 00Z ECMWF WEIGHTING WAS KEPT LOW ENOUGH TO
LEAN AWAY FROM ITS FARTHER NWD SYSTEM OFF KODIAK ISLAND, WITH A
SWD TREND OF THE GFS FROM 00Z TO 06Z THEN 12Z RUNS REINFORCING THE
MORE SRN CLUSTER.  THIS SOLN ALSO PROVIDED CONSENSUS DETAIL AND
INTERMEDIATE TRACK/ASPECTS OF SURROUNDING FLOW FOR THE COMPACT UPR
LOW INITIALLY JUST OFF THE NWRN CORNER OF THE MAINLAND.

FOR THE REMAINDER OF THE PERIOD INDIVIDUAL SOLNS TREND RAPIDLY OUT
OF PHASE FOR FEATURES ACROSS THE NRN PAC/ALEUTIANS/BERING SEA INTO
THE NERN PAC.  FOR EXAMPLE WITH AN EMERGING WRN PAC SYSTEM THE
00Z/06Z GFS WERE ON THE FAST SIDE OF THE SPREAD AND THE 00Z ECMWF
ON THE SLOW SIDE.  MOST 00Z GUIDANCE WEAKENED/PROGRESSED THE
INITIAL CNTRL ALEUTIANS SYSTEM TO A GREATER DEGREE THAN 06Z/12Z
GFS RUNS.  RECENTLY ARRIVED 12Z CMC/UKMET/ECMWF RUNS NOW ADD TO
THE POTENTIAL THAT THE ALEUTIANS/BERING SYSTEM MAY BE MORE
PERSISTENT THAN PREVIOUSLY FCST.  LATER IN THE PERIOD THE POOR RUN
TO RUN CONTINUITY IS MOST SIMPLY REPRESENTED BY THE PAST TWO ECMWF
RUNS, THE 00Z VERSION SHOWING A BERING SEA CLOSED LOW ALOFT AS OF
12Z SAT BUT THE 12Z/26 RUN DEPICTING A STRONG RIDGE.  GFS RUNS
SHOW SIMILAR VARIABILITY OVER THE PAST 24-36 HRS.  DOWNSTREAM THE
00Z GFS APPEARS TO HAVE THE LEAST PROBABLE EVOLUTION WITH A STRONG
SHRTWV REACHING THE SERN COAST/PANHANDLE AS OF 12Z SAT AND
SUBSEQUENT NEWD PROGRESSION AS A CLOSED LOW, RIGHT INTO THE MEAN
RIDGE DEPICTED BY THE MAJORITY OF GUIDANCE.  DUE TO THE RAPIDLY
INCREASING SOLN SPREAD, PREFERENCE QUICKLY ADJUSTS TOWARD THE 00Z
NAEFS/ECMWF MEANS AND 06Z GEFS MEANS DAYS 6-8 FRI-SUN AFTER A DAY
5 MODEL/ENSEMBLE MEAN TRANSITION.  THIS WOULD KEEP LOW PRESSURE
LINGERING A LITTLE S OF KODIAK ISLAND, WITH A POTENTIAL WWD SHIFT
IN OVERALL MEAN LOW PRESSURE LATE IN THE PERIOD WITH THE ARRIVAL
OF UPSTREAM ENERGY.  THE FCST PROVIDES REASONABLE CONTINUITY WHILE
WAITING FOR BETTER AGREEMENT ON SIGNIFICANT FEATURES, WHOSE
APPARENT TRENDS IN NEW 12Z GUIDANCE MAY OR MAY NOT PERSIST
CONSIDERING THE VARIABILITY/SPREAD SEEN THUS FAR.

RAUSCH

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